The invention relates to apparatus for detecting thermal neutrons, and more particularly, to an apparatus for detecting thermal neutrons which uses internal wavelength shifting optical fibers.
A standard method for detecting thermal neutrons is based on detection of the effects of secondary charged particles produced when a thermal neutron is captured by the helium-3 (.sup.3 He) nucleus. This reaction results in the production of a tritium (.sup.3 H) nucleus with a kinetic energy of 190 KeV and a proton (p) with a kinetic energy of 570 KeV. These energetic charged particles produce ionization tracks in surrounding substances. The ionization track will include ionized gas molecules (ions) which can be detected either by optical emissions or by direct collection of ions. Optical detection has the advantages over ion collection of more rapid response time and insensitivity to noise caused by vibration.
A neutron detector based on detection of light emission from a .sup.3 He gas mixture is described by Derzon, et al., in "A High-Pressure .sup.3 He Gas Scintillation Neutron Spectrometer," IEEE Transactions on Nuclear Science, Vol. 33, No 1, pp 247-249 (1986). The neutron detector of Derzon, et al., utilizes a pressure chamber filled With a mixture of .sup.3 He and Xe. The energy released by the reaction of a neutron with a .sup.3 He nucleus produces a pulse of ultra-violet (UV) light which is absorbed by a waveshifting fluorescent material which coats the inner surfaces of the pressure chamber. The fluorescent material then releases a pulse of visible light which is detected by a photomultiplier tube placed outside a window in the pressure chamber.
There are several disadvantages associated with the type of neutron detector described by Derzon, et al. For efficient light collection a large area window must be provided in the pressure chamber. This requires a large hermetic seal which is expensive to fabricate. Likewise, a large area optical detector such as a photomultiplier tube is needed. Large photomultiplier tubes are easily damaged. On the other hand, solid-state detectors with sufficient sensitivity, for example avalanche photodiodes, are only available with detecting areas up to about 1 cm.sup.2, while photomultiplier tubes are obtainable with areas of tens of centimeters squared. In order to use a small area photodetector, efficient means for collecting and concentrating photons must be provided.
Therefore, according to this inventive concept a need has been discovered for an apparatus for detection of thermal neutrons which uses a small area photodetector.